Squelch circuit



Feb. 26, 1963 F. J. SPENCER sQUELcH CIRCUIT 2 Sheets-Sheet 1 Filed Dec. 5, 1960 INVENTOR.

ATTORNEYS Feb. 26, 1963 F. J. SPENCER SQUELCH CIRCUIT 2 Sheets-Sheet 2 Filed Deo. 5. 1960 l EEE E Ummm IN V EN TOR. FRED J. 5P WW ATTORNEYS inite This invention relate generally to squelch circuits and, more speciiically, to squelch circuits for frequency modulation type receivers.

There are in the prior art many diierent types of squelch circuits which are designated for various applications. Some of these squelch circuits function to blank the receiver when the noise signal exceeds a certain level. Other squelch circuits operate in response to the level of signal-to-noise ratio. When the signal-to-noise ratio decreases to a point Where the signal is no longer intelligible the squelch circuit will function to blank the receiver until the Signal-to-noise ratio again increases to a point where the received signal becomes intelligible, at which time the rec iver becomes unblanked. ln all known sy-stems, however, there is an area between the blaniiing of the receiver and unblanking of the receiver where some of the intelligible signal is lost. More speciiically, for example, as the level o the signal-to-noise ratio or the received signal decreases, a point will be reached Where the squelch circuit will function to blank the receiver. Tow, in order for the receiver to become operative again, .e., unblanked, it is necessary that the signal-to-noise atio be increased to a value greater than the il (blank.- ng) signal-to-noise ratio. lt is apparent that ine intellince carried by the signal between the level of the nking signal-to-noise ratio and the level of the unnlzii g signal-to-noise ratio, will be lost to the receiver. Consequently, it is desirable to rnalre the unblanking level or" signal-to-noise ratio and the blanking level of signalto-noise ratio as nearly coincidental as possible.

in FM. receivers it is a characteristic th .reci that the noise power Will decrease with an increasi .g signal. Consequently, in an EM. receiver a squelch circuit can be designed which is a function of noise power. ln prior art circuits such a squelching function is effected by iirst extracting the noise signal from the received signal. Since the magnitude of said noise signal will increase as the signal strength decreases, a blanliing circuit, such as a Schmitt trigger circuit, can be made to blank the circuit when the noise level increases to a certain magnitude. On the other hand, as the Ri". signal increases in amplitude and the noise level decreases, a point will be reached 'where the Schmitt tr'cer circuit will function to unl-"leuk the re- There is, however, as indicated above,

ceiver circuit. an area between the noise level at which blanlring occurs and the noise level at which unblanl-:ing occurs wherein the intelligence-carrying RF. signal is not utilized. it will lbe apparent that less intelligence bearing RIF. signal will be lost if a greater change in noise level can be made to occur for a given change in the level.

it is an object of this invention lto minimize the amount of intelligence bearing RE. signal lost in the operation of a squelch circuit.

A further obiect of the invention is the improvement of squelch circuits, generally.

in accordance with the invention there is provided in an Fil/i. receiver, means for extracting the noise signal from the received signal, a blanlting circuit responsive to said extracted noise, and circuit means connected substantially in parallel with said blanking circuit and which provi es an impedance whose magnitude decreases as the signal-to-noise r tio increases; thus diverting an ever increasing portion of the noise signal from the 'olanlting patented Feb. 2d, i953 circuit as the RE. power increases, and thereby creating a largeincremental change in noise power for a given incremental change in power. The variable impedance circuit ineans comprises a transistor connected from the output of the noise extracting circuit to a reference potential (ground) through the collector-emitter circuit of said transistor. Said variable impedance circuit means also comprises rectit'yinf7 means which is responsive to the received signal lto produce a DC. potential whose magnitude is proportional to the signal plus the noise power (S-f-N) and to supply such DC. potential to the base electrode of the transistor. The aforementioned D C. potential functions to vary the impedance of the transistor in accordance with the magnitude of said D.C. potential. As the signal-to-noise ratio increases the signal plus noise power (S-l-N) will increase and the impedance of the transistor will decrease, thus diverting an increasing amount of noise signal therethrough. Thus it can be seen that the effect of the changing impedance of the transistor is to provide a greater incrementa change in noise signal (N) for a given incremental change in the signal-plus-noise (S-l-N), than would he the ase in the absence of the present invention. Wordcd in another manner, the clic-ct of the invention is to increase the change in the ratio S-l-N/N as .S-,LN increases.

The above mentioned and other objects and features of the invention will be more fully understood from the following etailed description thereof when read in conjunction with the drawings in which:

FG. l shows a fblocl; diagram of prior art structure;

FIG. 2 shows a combination block diagram and schematic ske-tch of the invention as applied to prior art structure;

PEG. 3 sh ws characteristic operating curves of noise .power and signal plus noise power as the El?. signal strength varies; and

PEG. 4 shows a schematic sketch or" an alternative form oi: the inveuti Referring now to FG. l, there is shown a portion of T ri`he output or" LF. amplifier lli is an nM. receiver'. supplied to an amplier limiter circuit il which amplies the received signal, including noise, and supplies the output thereof to the discriminator i3. The discriminator `l?. functions to extract the audio signal from the received signal in a well-known manner. Also, the discriminatori passes noise. The output of the discriminator l2 is supplied not only to the audio circuit 2li but also to the bandpass noise amplifier i3 which functions to pass and to amplify the noise appearing in a given frequency bandwidth usually lying near the edge of, or just outside, the band-width carrying ythe intelligence. The use of a noisecarrying bandwidth outside the intelligence bearing frequency bandwidth is permissible since the noise level in both frequency bandwidths is very likely -to be about the saine. The output signal ot the bandpass noise amplifier i3 is supplied to a suitable switching circuit, such as a Schmitt trigger i3 circuit which, in turn, functions in a conventional manner either to blank or to unblanlq the receiver, dept-endlosy upon lthe noise level. The blocks 5u and 5d contained Within the blanking circuits il and 4S of FlGS. l and 2, respectively, represent the input impedances or" said blanlring circuits.

Referring now to the curves of FIG. 3, the curve 3.6 shows the variation of noise level in volts which appears at the output terminal l5 of the band-.pass noise amplifier i3 of HG. l with variation in RE. strength, and the curve l? represents the variation of the niagni nde of the signal plus noise in volts appearing at the output l5 or" the amplitier l@ of FlG. l with variation in strength. From an examination of curves le and it will be apparent that the noise portion of the received signal comprises -this invention and is shown in FIG. 2.

substantially all of the received signal until the RE. portion of vthe received signal attains a value or about .7 microvolt; at which time the noise generated in the receiver begins to decrease sharply, thus permitting an increasingly greater incremental change in noise as the signal-plus-noise (S-l-N) increases. As discusse hereinbefore, such incremental change in noise level is employed to operate an appropriate circuit such as a Schmitt trigger circuit which, in turn, functions to blank or unhlanlt the receiver.

Tue noise level at which tde circuit becomes unblanlted is designated as em, and the noise level at which the receiver circuit lbecomes blauired is designated as em. The dierence between en, and eng is designated as den and represents the differential noise voltage recuired to operate the Schmitt trigger circuit. Anexamination of PiG. 3 will show that the change in RE. power required to pro duce the dierential noise voltage equal to den is dlerf. In more speciiic terms, it is required that the ELF. power increase to a magnitude of about l.7 microvolts before the Schmitt trigger circuit will operate to unblanl; the circuit (assuming the receiver was in a blanked condition originally). Then, to blank the receiver, the RSE. voltage must decrease to a magnitude of about .7 microvolt. The difference between .7 and 1.7 microvolts represents a loss of intelligible signal covering an RF. range of 1 microvolt. It is, perhaps, appropriate to point out at this time ,that the principal obiect of this invention is to reduce, as much as possible, this l microvolt range of RF.

signal which is lost to the receiver.

The curve 18 of FiG. 3 shows a modiiied noise characteristic curve in which the noise voltage tolerance den ,is obtained with an incremental difference of RJ?. volage equal to about .30 microvolt, which Ais represented by the voltage Azeri in FiG. 3.

The circuit means by which the modiiied noise characteristic curve 18 is obtained comprises the subject of Suchcircuitry is comprised of rectifier 2S, an integrating circuit including resistor it? .and capacitor dl, current limiting impedance 26, transistor 27, and impedance 2S. A. voltage source 32 supplies collector voltage to the transistor 27 through impedance 33. Blocks lo', 4S', il', 12', 13', and Ztl correspond to the blocks similarly Videntified (without primesl in FiG. l.

In FIG. 2 the collector electrode 34 of transistor 27 is .connected to the output of the bandpass noise arnplier 13 and the emitter electrode 35 is connected to ground potential through impedance 23, which can be a resistor.

Rectifier has its cathode connected to the output terminal i5 of LF. amplilier 1%' and will produce, at point 42, in cooperation with the integrator circuit comprised .ori resistor Il@ and capacitor el, a D.C. voltage whose magnitude varies proportionally as the signal-plus-noise (S+N). The signal-plus-noise voltage is represented by the curve 7170i FIG. 3. The said D C. voltage is supplied through the current limiting impedance 26 to the base electrode 43 of the transistor 27.

The DC. voltage supplied to the base electrode 43 of transistor 27 wilunction to change the conductivity of the transistor 27 in proportion to the magnitude thereof. Referring now to the curve 17 of PEG. 3, it can be seen that if the RE". input increases beyond a magnitude of about .7 microvolt the signal-plus-noisc level begins to increase rapidly and, concurrently, the conductivity of the transistor 27 `will become increasingly greater. Also, concurrently, the amplitude of the noise signal will be decreasing as indicated hereinbefore. However, due to the fact that the conductivity of the transistor Z7 is increasing, an ever increasing amount of the noise will be .diverted from point -lf thro-un the transistor 27 and impedance 2S to ground potential. The resulting noise signal characteristics appearing at terminal 14' is represented by the portion l@ of curve ld of Fifi. 3. It vwill be observed that the rate of change of the amplitude of the noise signal of the portion i9 of curve i8 during said interval is considerably greater than `the rate of change oi t e corresponding portion of noise curve 16, said last-mentioned curve representing the operation of the circuit in the absence ot transistor 27 and its associated circuitry. it can ne seen, consequentl, that the ecessary differential noise signal den required for blanking and unblanlring the circuit is obtained with considerably less loss of signal.

Referring now to FiG. 4, there is shown an alternative form o the invention in which elements which have corresponding counterparts in FiG. l and FG. 2 are identitied with the same reference characters (although primedl. To avoid repetition, the entire circuit of FIG. Z is not reproduced in FIG. 4, but only that portion of the circuit between the junctions 15 and i4.

The principal diierence is the structure of FIG. 4 is that a PNP type transistor is employed, whereas in the structure of FlG. 2 an NPN type transistor is employed. ln accordance with the use of a ENP type transistor in FiG. 4 the diode its is connected with a polarity reverse to that of diode of FiG. 2. Thus, a negative D C. votlage will be produced at the base electrode of the ransistor 47 to vcontrol its conductivity. More specifically, as the RF. signal .strength increases the signal-plusnoise voltage of curve i7 of FIG. 3 will increase and the negative D.C. votlage supplied ,to the base electrode of transistor .6:17 will increase, whereby the conductivity of transistor t7 will increase. The increased conductivity of transistor @7 will function to divert an increasing amount of the noise signal from the output terminal 14" of the bandpass noise amplifier. It is to be understood that the portion of the circuit of FIG. 2 not shown in FIG. 4, is a part of the circuit of FIG. 4.

it is to be noted that the forms of the invention shown and described herein are but preferred embodiments thereof and that various changes-may be made in the circuit arrangements and the application thereof without departing from the spirit or the scope of the invention.

l claim:

l. in a radio receiver comprising means for receiving a signal containing au intellience bearing signal and noise signal, means for extracting from the received signal a direct current signal whose magnitude varies proportronately with the average power of the received intelligence bearing signal-plus-noise signal, means for extracting the noise signal from said received signal, blanking means having an input means and constructed to be responsive to said extracted noise signal for blanking said radio receiver When the amplitude of said noise signal exceeds a predetermined level, and diverting ,means cornprising a variable impedance and connected in parallel with input means of said blanking means, said variable im` edancc constructed to be responsive to said direct curre t signal to divert from said blanking signal a portion of said extracted noise signal, which portion varies in accordance with the magnitude of said direct current signal.

2. A radio receiver in accordance with claim l in which said variable impedance comprises an electron valve having an electron emitting electrode, an electron collecting electrode, and an electron control electrode, in which said diverting means comprises impedance means connected in series arrangement with the electron emitting lectrode-electroncollecting electrode circuit of said electron valve to form the parallel circuit with the input means of said blanlting means, and means for supplying said direct current signal to said electron control electrode to control the electron collecting electrode-electron emitting electrode circuit impedance of said electron valve.

3. A radio receiver in accordance with claim 2 in which eens tor extracting said direct `current signal from the .received signal comprises rectitying means to detect. .the received Ysignal and integra ing means constructed to www be responsive to said detected signal to produce said direct current signal.

4. A radio receiver in accordance with claim 3 in which said electron valve is a PNP type transistor, and in which said rectifying means is constructed to produce a negative direct current voltage with respect to the potential of said electron emitting electrode.

5. A radio receiver in accordance with claim 3 in which said electron valve is an NPN type transistor, and in which said rectifying means is constructed to produce a positive direct current voltage with respect to the potential of said electron emitting electrode.

6. A frequency modulation type radio receiver comprising means for receiving a signal containing an intelligence bearing signal and noise signal, means for extracting from the receivedsignal a direct current signal Whose magnitude varies proportionately with the average power of the received intelligence bearing signal plus the noise signal, rst means including a baudpass amplifier for eX- tracting noise signal from said received signal, blanking means including an input means and constructed to be responsive to said extracted noise signal for blanking said radio receiver when the amplitude of said noise signal exceeds a predetermined level, and diverting means having a part thereof including a variable impedance connected in parallel with the input means of said blanking means, said variable impedance constructed to be responsive to said direct current signal to divert from said blanking signal a portion of said extracted noise signal, which portion varies in accordance with the magnitude of said direct current signal,

7. A radio receiver in accordance with claim 6 in which said variable impedance comprises an electron valve having an electron emitting electrode, an electron collecting electrode, and an electron control electrode, and means for supplying said direct current signal to said electron control electrode to control the electron collecting electrode-electron emitting electrode circuit impedance of saidelectron valve.

8. A radio receiver in accordance with claim 7 in which said means for extracting said direct current signal from the received signal comprises rectifying means to detect the received signal and integrating means constructed to be responsive to said detected signal to produce said direct current signal.

9. A radio receiver in accordance with claim 8 in which said electron valve is a PNP type transistor, and in which said rectifying means is constructed to produce a negative direct current voltage with respect to the potential of said electron emitting electrode.

l0. A radio receiver in accordance with claim 8 in which said electron valve is an NPN type transistor, and in which said rectifying means is constructed to produce a positive direct current voltage with respect to the potential of said electron emitting electrode.

References Cited in the tile of this patent UNITED STATES PATENTS 2,425,968 Tuniek Aug. 19, 1947 2,632,101 Quarles Mar. 17, 1953 2,704,324 Broadhead Mar. l5 l 1955 

1. IN A RADIO RECEIVER COMPRISING MEANS FOR RECEIVING A SIGNAL CONTAINING AN INTELLIGENCE BEARING SIGNAL AND NOISE SIGNAL, MEANS FOR EXTRACTING FROM THE RECEIVED SIGNAL A DIRECT CURRENT SIGNAL WHOSE MAGNITUDE VARIES PROPORTIONATELY WITH THE AVERAGE POWER OF THE RECEIVED INTELLIGENCE BEARING SIGNAL-PLUS-NOISE SIGNAL, MEANS FOR EXTRACTING THE NOISE SIGNAL FROM SAID RECEIVED SIGNAL, BLANKING MEANS HAVING AN INPUT MEANS AND CONSTRUCTED TO BE RESPONSIVE TO SAID EXTRACTED NOISE SIGNAL FOR BLANKING SAID RADIO RECEIVER WHEN THE AMPLITUDE OF SAID NOISE SIGNAL EXCEEDS A PREDETERMINED LEVEL, AND DIVERTING MEANS COMPRISING A VARIABLE IMPEDANCE AND CONNECTED IN PARALLEL WITH INPUT MEANS OF SAID BLANKING MEANS, SAID VARIABLE IMPEDANCE CONSTRUCTED TO BE RESPONSIVE TO SAID DIRECT CURRENT SIGNAL TO DIVERT FROM SAID BLANKING SIGNAL A PORTION OF SAID EXTRACTED NOISE SIGNAL, WHICH PORTION VARIES IN ACCORDANCE WITH THE MAGNITUDE OF SAID DIRECT CURRENT SIGNAL. 